Certain atomic nuclei, typically having a spin quantum number of 1/2, exhibit magnetic signatures when they are within an externally applied magnetic field. This magnetic resonance effect is most commonly observed in .sup.1 H, and is known as nuclear magnetic resonance (NMR). Atomic nuclei with a spin quantum number of &gt;1/2 can also show another magnetic signature associated with the interaction of the nuclei with the local electric field. This phenomenon is known as nuclear quadrupole resonance (NQR).
For both of these phenomena, the energy level transitions are observed primarily in the radio frequency range. Detection of these transitions thus requires a radio frequency source to excite the transition, and a radio frequency receiving mechanism to detect the signal. Normally, the signals appear at a pre-defined frequency. An RF coil tuned to, or close to, that predefined frequency can excite or detect those signals. The signals are of very low intensity and can only be observed for a short time. approximately 10 .mu.s to 2 ms. As a consequence, there is a need for an NQR or NMR receiver that can be tuned to (usually) high Q, has very low noise, and is capable of fast recovery after a high voltage RF pulse. In most conventional magnetic resonance (NMR and NQR) experiments, small and fairly homogeneous samples are investigated.
Over the past few years there has been considerable interest in the larger-scale "real world" applications of both NQR and NMR. These applications do not benefit from the luxury of small-scale laboratory investigations. They usually require investigation of large volumes filled with materials of vastly differing physical and chemical composition. Investigation of the contents of mail or baggage for the presence of explosives or narcotics is an example.
With respect to explosives, plastic explosives such as C-4 and Semtex, containing RDX and PETN, have an almost infinite variety of possible shapes and uses for terrorist bombing tactics. Plastic explosives are highly stable, have clay-like malleability and are deadly in relatively small quantities. A small piece of plastic explosive, a detonator, and a trip wire inside a large mailing envelope can cause a deadly explosion. Unfortunately, without close--and potentially dangerous--visual inspection, plastic explosives can be made virtually untraceable. In particular, detection of sheet explosives, typically having a thickness as small as one-quarter inch, has not been effectively accomplished by prior technologies.
The wide-scale attempts to fight the illegal drug trade indicates that narcotics detection is also extremely important. The need for a simple procedure for detecting drugs inside sealed containers, mail parcels, and other small packages, quickly and accurately, is immeasurable. Conventional detection methods are time-consuming, costly, and have only marginal reliability at best.
Detection by means of NQR or NMR is possible for both explosives and narcotics, partially because they have as a constituent element .sup.14 N in crystalline form. Particularly with respect to narcotics, this is true of cocaine base, cocaine hydrochloride and heroine based narcotics. The hydrochloride forms of narcotics, such as cocaine hydrochloride, also contain quadrupolar nuclei .sup.35 Cl and .sup.37 Cl.
A significant factor in contraband detection by means of NQR in particular is that quadrupolar nuclei that are commonly present, and potentially readily observable, in narcotics and explosives include nitrogen (.sup.14 N) and chlorine (.sup.35 Cl and .sup.37 Cl), among possible other nuclei. Thus, in commercial applications it is necessary to be able to detect quadrupolar nuclei contained within articles of mail, mail bags or airline baggage, including carry-on and checked luggage. While the resonant frequencies of the nitrogen in these substances differs for each chemical structure, these resonant frequencies are well defined and consistent. By applying an RF signal to a container having any of these suspected substances inside, and then detecting any quadrupolar resonance thus engendered by the application of RF pulses, the identity of the contraband substance can be easily determined.
NQR and NMR signals originate from the energy transitions associated with certain nuclei. These energy transitions are usually in the radio frequency range. Thus, detection of both NQR and NMR signals normally requires the use of radio frequency transmitting and receiving apparatus. To minimize noise and radio frequency power requirements and improve receiver sensitivity, conventional NQR and NMR systems use a narrow band, high Q, sample coil for both transmitting and receiving. There are, however, a number of factors that can significantly degrade the effectiveness of detecting NQR and NMR signals using this kind of narrow band, high Q, detection apparatus. Some of them are:
(1) the presence of large conductive materials inside the sample coil;
(2) the presence of materials with high dielectric constant inside the sample coil;
(3) temperature, which can affect the value of the capacitance used for tuning and matching the RF coil; and
(4) mechanical movement of the coil with respect to its surroundings. All of these factors can cause serious de-tuning of the detection apparatus, which in turn causes a reduction in the detection sensitivity of NQR and NMR signals from the materials inside the sample coil.
Previously, for most applications of NQR and NMR, these conditions have not presented a serious drawback. The apparatus could usually be set up under near-optimum conditions, and the materials being investigated were usually well characterized. However, over the past few years several new applications have arisen which require NQR and NMR apparatus and methods for the detection of certain materials under adverse conditions (for instance, applications in which large volumes of largely uncharacterized materials are under investigation).
A system intended for use in nuclear resonance experiments involving frequency sweeps is described in Butler et al.. High-Power Radio Frequency Irradiation System with Automatic Tuning, Rev. of Sci Instruments, Vol 53, No. 7, pp 984-988 (1982). This reference teaches adjusting the frequency of the system to a known frequency by means of a look up table. A multi-turn tuning coil wound on a glass mandrel is employed.
U.S. Pat. No. 5,208,537 is directed at a method for matching antennas in an NMR imaging apparatus. Like Butler above, this reference builds a look up table specific to each patient for which a tomogram is being produced.